1. Field of the Invention
This invention relates generally to digital communication networks and, more specifically, to apparatus for providing highly reliable, substantially error-free communication on T1 links for use in, preferably, private communication networks.
2. Description of the Prior Art
Since the introduction of time-division multiplexed signals into North American telecommunication networks in the 1960's, a sophisticated time-division multiplexed signal hierarchy has evolved based on a so-called T1 format. This format represents a type of transmission scheme that is capable of supporting 24 time-division multiplexed voice channels organized into 24 time-slots on a single physical transmission facility or T1 link. In the traditional T1 format, a 64 Kbps signal carried by each time-slot represents each voice channel. However, lately the T1 format is increasingly used for digital data transmission as well as the traditional voice channel transmission. As such, each 64 Kbps signal carried by each time-slot can also represent a data channel. Both digital and analog facilities are used to propagate T1 formatted time-division multiplexed signals. Typically, a digital communication link or facility which carries T1 formatted signals is known as a DS1 link. As such, the term DS1 refers only to the digital signals in the T1 format.
Generally, T1 links are designed as having a network topology that is either point-to-point or point-to-multi-point. In the point-to-point network topology, all 24 channels that originate at one customer site terminate at a single customer site at a far end location in the network. By comparison, in the point-to-multi-point network topology, not all 24 channels that originate at one customer site terminate at a single customer site at a far end location in the network. In other words, the 24 channels originating at one customer site may be destined for multiple far end customer sites. Typically, telecommunications system customers will use both types of topologies to exchange data with various geographically diverse sites.
Customer premise equipment known as protection switches are used for special applications that require enhanced transmission link availability. The protection switch used in a point-to-point network topology is known as an error correction switch (ECS). The protection switch used in a point-to-multi-point network topology is known as a DS1/0 protection switch (DS1/0 PS). In general, each protection switch type utilizes redundant T1 communication links, actively monitors these links, and, upon failure of any of these links, provides automatic restoration of telecommunication services using a back-up T1 link, i.e. one of the redundant links.
Before discussing the details of the protection switch circuitry used in the art, it is necessary to trace pertinent aspects of the evolution of digital transmission networks as they are used in each topology so as to provide a context for presenting and discussing the relevant art. First, the point-to-point network topology is described, followed by a description of a point-to-multi-point network topology. To enhance understanding, each such description includes a discussion of associated protection switch circuitry.
In the original T1 format used in point-to-point network topologies, 24 voice-frequency signals are sampled and then multiplexed into a bit stream, thereby forming 24 autonomous voice-frequency channels known as DS0's or DS0 channels. Each voice-frequency channel is allocated 8 bits in each T1 frame; these 8 bits represent one encoded sample of a particular voice signal as well as signaling information. Specifically, seven out of eight DS0 bits in each slot are used to represent encoded voice with one additional bit being used exclusively for signaling. All 24 channels are grouped together and arranged in 24 sequential time-slots to form a 192-bit data group. For synchronization, every group of 192 bits is preceded by a framing bit. The resulting 193 bits comprise a T1 frame. Thus, a T1 frame is composed of 192 bits of information and one overhead framing bit. Consequently, since the sampling rate of the voice- frequency signal is 8 kHz, meaning that each voice- frequency signal must be sampled 8000 times a second, the transmission rate of the T1 system is 1.544 Mbits/sec. To support two-way communication, two digital bit streams are generated--one for each direction of transmission. Each bit stream is transmitted over a corresponding unidirectional link typically composed of a wire-pair cable and interposed repeaters so that a complete T1 system utilizes two wire-pair cables. At each end of the wire-pair cables, T1 terminals known as D1 channel banks, were used to multiplex the 24 voice-frequency signals.
As technology evolved, additional framing formats were devised to improve the performance of the underlying T1 system. For example, the D2 frame format introduced the so-called superframe wherein 12 contiguous T1 frames were sequentially grouped to form the superframe. A unique 193rd bit sequence was repeated once every 12 frames, i.e., for each superframe. The D2 framing format also introduced so-called "robbed bit" signaling. Specifically, in each DS0 channel, only one bit, just in frames 6 and 12, was used for signaling. The remaining bits were allocated to encode customer voice. The use of all eight bits for digitized voice in 10 out of 12 frames resulted in substantial improvement of voice transmission quality compared to the D1 framing format. D1D, which followed D2, made the superframe concept "backward compatible" with the original D1 type channel banks. The last frame format to utilize a 12-frame superframe concept was the D4 frame format. The most recent framing format, designated the Extended Superframe Format (ESF), dramatically improves the capabilities of T1 systems and affords an opportunity to perform in-service end-to-end error performance monitoring on working T1 systems. Because of technology improvements, fewer bytes are now required to maintain frame synchronization as compared to the D4 framing format. In ESF, a 24-frame sequence is used for framing, in-service, end-to-end performance monitoring, and facility data link (FDL) that allows communication on the T1 link independent of the customer data traffic. FLD channel has been designed to allow the carrier to provide in-service maintenance of T1 facilities. The fundamentals of ESF will be discussed in more detail shortly after the types of errors that arise in a T1 system are discussed.
A T1 bit stream, that is, a stream of T1 frames, must follow certain propagation rules. First, when bits of the T1 signal propagate over the communication medium, each "1" bit must correspond to a pulse having a polarity opposite to the polarity of the previous "1" bit--this format generates a bipolar signal composed of a stream having alternating positive and negative electrical pulses. For each "0" bit, no electrical pulse is propagated. Such a bipolar signal leads to spectral efficiency, and also results in a frequency spectrum lacking a DC component, thereby requiring only AC-coupled circuitry. Second, to maintain signal synchronization, no more than a certain number of "0" bits can be transmitted consecutively. A logic error occurs if a "1" bit appears where a "0" bit should occur, and vice versa. A format error occurs when there is a violation in the bipolar nature of the pulse stream, an error in the framing bits, or an excessive number of "0" bits occur.
For all frame formats existing prior to the introduction of ESF, e.g., the D4 framing format, in-service performance monitoring was limited to checking for format errors. The basic assumption was that any logic error caused a format error. However, only certain logic errors cause format errors. For instance, if two consecutive "1" bits in a bit stream, e.g., a positive pulse followed by a negative pulse, are converted to "0" bits, then no format errors occur since there are no bipolar violations. However, two logic errors have occurred. In addition, a typical T1 circuit is likely to regenerate bipolar signals a number of times before the signal reaches a destination receiver. For example, a higher order M13 multiplexer combines several T1 signals into a T3 fiber link. Since fiber does not carry bipolar signals, the M13 multiplexer must regenerate the bipolar signals each time the signal from the fiber is demultiplexed from T3 to T1. Upon demultiplexing, any bipolar violations that may exist are "corrected". As a result, a channel bank at the other end of the T1 link can monitor bipolar violations, but this will reveal only bipolar violations that occurred in the last leg of the path from the last M13 multiplexer (i.e., bipolar pulse source) to the channel bank. Thus, end-to-end error performance monitoring cannot be provided with D4 framing format. Obviously, a need has existed for some time to develop a technique which could accurately detect end-to-end performance of an in-service T1 system.
This need was satisfied by the development of the ESF concept. With respect to the principles of ESF, each superframe is defined as encompassing 24 T1 frames, such that each ESF superframe is composed of 24.times.193=4632 bits, including 4608 information carrying payload bits and 24 overhead bits in the 193rd bit position. T1 frames of a conventional T1 system are conveyed at an effective rate of 8000 bps. Accordingly, the 24 overhead bit positions in the ESF superframe occupied by the 193rd bit in each of the 24 underlying T1 frames are also conveyed at the same rate of 8000 bps.
However, because of the aforementioned advances in technology, ESF superframe synchronization requires only six of the 24 overhead bits. Thus, if the 24 ESF overhead bits are extracted from the 4632-bit ESF superframe and arranged serially to create an ESF framing bit sequence, then the ESF superframe synchronization bits are conveyed by every fourth bit in the ESF framing bit sequence, that is, in an ESF superframe synchronization sub-sequence. The ESF superframe synchronization bits have a fixed bit pattern which conventionally has been set at "001011". It is important to reiterate at this point that the framing bit sequence is an overhead portion of the ESF superframe that is used to carry user information data and signaling data. Additionally, the information bits are unaffected by what is being conveyed in the ESF framing bit sequence.
The remaining 18 bits in the ESF frame sequence are partitioned into a check sub-sequence of 6 bits and a facility data link (FDL) sub-sequence of 12 bits. The check sub-sequence is of particular relevance to the present invention.
As indicated above, the primary reason for devising the ESF technique was to enable T1 users/suppliers to accurately measure end-to-end error performance of a T1 system while the system is on-line, that is, without interrupting propagation of the T1 bit stream. This is accomplished by applying a coding theory technique, called Cyclic Redundancy Codes (CRC), to each ESF superframe. With the CRC technique, all 4608 payload bits in each complete ESF superframe can be checked for logic errors. In essence, for the particular CRC technique used in the ESF (called CRC-6), six bits are generated for each ESF superframe; these six bits are indicative of the actual 4608 information bits comprising a given ESF superframe.
Computation of the six check bits merely involves modulo-2 division. For example, assume that the ESF superframe, rather than containing 4608 information bits, contains 10 bits and the pattern is "1010101010", which has a decimal equivalent of 682. This decimal equivalent is divided by "111111" which is a decimal equivalent of 64 (2.sup.6 =64, that is, 6 bits are dedicated to the check subsegment), resulting in a decimal quotient of 10 and a decimal remainder of 42. The quotient is ignored, and the six-bit binary equivalent of the remainder, i.e., 42, namely, "101010" are the check bits transmitted in the next superframe. For ease of understanding, the foregoing example used base 10 arithmetic to generate the check bits. In practice, the check bits are generated using modulo-2 arithmetic.
To visualize the flow of bits in a temporal sequence, consider a time sequence of two ESF superframes designated the first ESF superframe and the second ESF superframe, wherein the latter superframe follows the former in time. The six check bits computed for the first ESF superframe are entered into the check sub-sequence of the second ESF superframe and transmitted to the receiving end of the T1 system. At the receiving end, the CRC-6 code of the first ESF superframe is dynamically computed. Obviously all 4608 information bits of the first ESF superframe are needed to compute the code bits, so the CRC-6 code cannot be completed until all bits arrive at the receiving end. Once computed, the CRC-6 code bits can then be compared to the check sub-sequence arriving in the second ESF superframe. If a match occurs, then it is concluded with a high degree of confidence that there were no errors in the information bits contained in the first ESF superframe. If there is not a match, this is normally taken to mean that at least one logical error occurred in the information bits of the first ESF superframe. (Of course, the check bits in the second superframe may have been corrupted, but the probability of this occurring, i.e., errors in the six bit positions, is much less than the occurrence of errors in 4608 payload bit positions.) The unfolding nature of error checking for the general case is now readily discernible: the CRC-6 code bits of a given superframe are transmitted in the check sub-sequence of the next superframe following the given ESF superframe.
Representative of prior art devices and equipment which exploit the capabilities of error detection using CRC-6 codes with ESF superframes is the Automatic Protection Logic Switch (APLS) available from the Verilink Corporation located in San Jose, Calif. The APLS checks a working T1 line for logic errors, and when a preset Bit Error Rate (BER) threshold is reached or an active link outage is detected, traffic is automatically switched from the active T1 link to a standby T1 link. Such an arrangement is deficient, however, because the protection link (stand-by link) is activated only after errors occur and the BER threshold is reached. Consequently, all errors which occurred in the active link prior to the activation of the protection link reach the end-user. Also, a difference in the propagation delay exists between the active and protection links. When the protection link is activated, the end-user will encounter a "hit" in the data stream; this "hit" is usually in the form of a gap in the data stream or a repeated segment in the data stream.
Recent developments in the art have produced an arrangement and concomitant methodology which equalizes the propagation delays of an active T1 link and a protection T1 link and utilizes the protection T1 link to correct errors in the active T1 link. The equalization of the propagation delays achieves "hitless" protection switching when the active T1 link fails--as distinct from the sequence in the prior art of error-detection first, then subsequent protection switching to the alternate facility only after a period of delay. One such "hitless" protection switch known as an Error Correction Switch (ECS) is disclosed in United States patent application entitled "Intelligent Digital Signal Hitless Protection Switch" filed Apr. 7, 1993, and accorded Ser. No. 08/044,348, and which is incorporated by reference herein and assigned to the present assignee hereof.
Due to the manner in which the CRC-6 coding is computed, i.e., a single code corresponds to all 4608 information bits carried by an ESF superframe, the protection switches that capitalize on the existence of the CRC-6 coding in the ESF superframes are generally limited to use in point-to-point T1 telecommunications links between individual customer sites. Typically, dedicated hardware, such as the "hitless" protection switch discussed above, is used to provide enhanced availability and error correction of T1 links. However, the hardware and software used to provide protection for point-to-point T1 communications is typically not equivalent to the hardware and software used to provide protection for point-to-multi-point T1 communications. Thus, as discussed below, separate and independent protection hardware is typically used at each customer site in point-to-multi-point network topologies.
As alluded to above, in addition to having a point-to-point T1 communications link between two customer sites, many telecommunication customers utilize additional T1 access facilities at a customer site to communicate with multiple customer sites, i.e., a point-to-multi-point network topology. In such point-to-multi-point network topologies, the 24 individual voice/data channels comprising a T1 formatted frame that originate at one customer site may terminate at multiple customer sites at the far end. To ensure high channel availability, point-to-multi-point topology uses redundant and diverse T1 network facilities. Protection switch hardware at each end of a protected channel, i.e., a link between two customer sites, monitors both active and alternate routes for channel outages and selects a working route for communication. To understand the nature and usefulness of channel protection switches (also known as DS1/0 protection switches) for such T1 redundant links, a review of the operation of point-to-multi-point T1 networks is warranted.
With modern telecommunications networks, a customer such as a business customer may select from an array of communication services aimed at providing cost-effective connections to geographically-dispersed sites each having independent data sources. The various available communication systems alternatives range, at one extreme, from ubiquitous direct dialing over the public network to, at the other extreme, specially provisioned private networks. Because of the vagaries of direct dialing, such as call blocking and connect-time cost, a customer with critical communication requirements most often selects a private network. In particular, there has recently been a demand for large private data networks to connect, for example, numerous terminal devices such as reservation terminals to an arrangement of centrally located, fault-tolerant host computers and thereby service consumer transactions. Data networks are generally implemented on private network facilities because computer applications necessitate continuous, on-line connections of terminals to centralized computers.
One common method of implementing a private line network is to connect various customer sites to a T1 network with T1 digital access facilities wherein numerous channels are multiplexed to generate a T1 signal suitable for carriage over these facilities. A channel may have more than one DS0 (i.e. N.times.DS0, where 1.ltoreq.N.ltoreq.24). A DS0 is a single 64 kbs signal embedded in the T1 transmission. In reference to point-to-multi-point network topologies, each channel embedded in the T1 transmission will hereinafter be referred to as a fractional T1 channel. Each fractional T1 channel transfers one or more DS0's between two customer sites. At a near end, a number of fractional T1 channels from one or more sites are multiplexed to form a T1 signal. At a destination end (far end), the high-rate T1 signals are demultiplexed to recover the fractional T1 channels. Oftentimes, fractional T1 channels are provided by a common carrier over carrier-owned T1 facilities on a long-term or semi-permanent basis. T1 facilities are provisioned by the common carrier through static cross-connect switches generally referred to as a Digital Access and Cross Connect System (DACCS). Unlike telephone-carrier switches which handle telephone call setups, a DACCS establishes routes which may remain connected for years.
Efficient use of T1 facilities requires individual routing of fractional T1 channel. Therefore, fractional T1 channels within the T1 signal which are terminated at a particular customer site, may not all be derived from the same customer site at the far end. Typically, facility failures in such private networks require restoration efforts that are correspondingly sophisticated and time-consuming. Accordingly, facility engineers have sought techniques to provide for efficient and automated restoration of facilities when deleterious service conditions are detected.
Recently, a service has been introduced by one inter-exchange carrier which provides switched digital data service at fractional T1 speeds. In using the service, a customer is able to establish a back-up dial-up link to restore a failed fractional T1 channel. However, such a service has the disadvantage of blocking, that is, a link between the end points is not always available. Moreover, it takes at least a few seconds to establish a connection over such a link each time a dial-up is attempted. A significant loss of data can occur within a few seconds.
To improve channel restoration capabilities in T1 communications networks, a protection switch as disclosed in United States patent application entitled "Automatic Restoration of Fractional DS1 Channels Carried Over DS1 Digital Links", filed Dec. 9, 1991 and accorded Ser. No. 07/805,340, which is incorporated by reference herein and assigned to the present assignee, has been developed. This arrangement, known as a DS1/0 protection switch, hereinafter DS1/0 PS, uses an active and a back-up communication path to propagate redundant, fractional T1 channels.
Generally speaking, to implement the channel restoration apparatus, a pair of DS1/0 PS are situated at near and far ends of a protected fractional T1 channel. A protected fractional T1 channel is comprised of two fractional T1 routes, one active and one inactive (alternate), both of which propagate identical data. Each DS1/0 PS includes: (1) detection circuitry to detect the presence of a DS0 fault indication signal, such as a digital access cross-connect system (DACCS)-generated trouble code, in any of the DS0's of the fractional T1 channel arriving over either an active or an alternate network; and (2) switching circuitry to transfer the corresponding fractional T1 channels having the trouble code in at least one of its DS0's from the active route to the alternate route. The use of paired devices spanning both ends of the fractional T1 channel ensures that both directions of propagation are protected from fault conditions that could arise within either network and adversely affect transmission in either direction over either one of the networks. In addition, each DS1/0 PS also includes circuitry to transmit two identical versions of data, generated by terminal equipment, which then are transmitted to the far-end over both the active and alternate routes. Each DS1/0 PS operates by detecting a so-called DACCS "trouble code" which is inserted by a DACCS on all outgoing DS0's at the occurrence of a T1 facility failure. Subsequently, based upon the presence of this code, the DS1/0 PS automatically switches a fractional T1 channel from an active route to a corresponding fractional T1 channel in the alternate fractional T1 route provided by the alternate network in order to restore service.
Both the ECS and DS1/0 PS are produced as dedicated, single-purpose hardware. Typically, each switch operates in conjunction with individual support circuitry and software. To reduce costs, it would be desirable to combine both types of protection switches into a single hardware unit. Moreover, utilization of two separate switches having separate chassis and support circuitry, disadvantageously requires a significant amount of space within customer premises. As such, a need currently exists in the art for a combination protection switch that operates in: (a) one mode for protecting point-to-point T1 links while at the same time utilizing a protection T1 link to correct errors in an active T1 link; (b) and another mode for protecting fractional T1 channels.